1. Field of the Invention
The present invention relates to a tape cassette including a casing which accommodates a tape reel wound with a tape and a cover member for protecting the tape, and more particularly to a tape cassette having front, upper and rear covers as cover members.
2. Related Background Art
A tape cassette including three pieces of cover members, i.e., front, upper and rear covers, has thitherto been known as, e.g., a DVC (Digital Video Cassette). In this type of tape cassette, the tape exposed outside the cassette in front of the casing is protected by being covered with the front, upper and rear covers when unused. FIGS. 4A and 4B are side views each showing the conventional cassette tape. As illustrated in FIGS. 4A and 4B, a front cover 103 is rotated when using the tape cassette 101, and an upper cover 104 and a rear cover 105 move interlocking with the rotation of the front cover 103 and thus open the front surface of a casing 102.
As indicated by a broken line in FIG. 4B, when the front cover 103 rotates upward, the upper cover 104 rotates about rotary spindles 106, and moves rightwards in FIG. 4B while being guided at its guide spindles 104a along a guide groove 107 of the casing 102. Simultaneously, the rear cover 105 rotates about its rotary spindles 105a and moves upwards while being guided at its guide spindles 105b along a cam groove 108 of the casing 102. As shown in FIG. 4A, when the front cover 103 rotates farther upwards, the front surface of the casing 102 opens, and at the same time the tape 100 is released from a state of being protected by the front, upper and rear covers. At this time, a front side end of the rear cover 105 moves while depicting a trajectory x by a one-dotted chain line in FIG. 4A.
As to a configuration of the guide groove for guiding the upper cover described above, Japanese Patent Application Laid-Open Publication No. 8-87861 discloses that the upper cover slides easier with the guide grooves having a smaller inclined angle, however, it is undesirable that the rear cover, if under 35 degrees, interlocking therewith might interfere with the tape.
Japanese Patent Application Laid-Open Publication No.11-110947 discloses that a slide resistance caused by opening the cover member decreases when an opening angle of the front cover is within a range of 0 to 30 degrees, and gives a specification value for a relation between an opening angle of the front cover and a height of the rear cover when the cover member of the tape cassette opens (FIGS. 18 and 19).
As described above, in the tape cassette having the cover body composed of three pieces of the cover members, the cover body is easy to open, but there are problems in terms of the interference between the rear cover and the tape and a restriction based on the specification requirement of the height of the rear cover when the cover body opens. It is required that those factors be taken and a graph, wherein there is given a specification value of a height of the rear cover 105 from a bottom surface of the casing with respect to a rotation angle .gamma. of the front cover 103 in FIG. 4B. In the tape cassette in the prior art, the rear cover has a height C satisfying the specification requirement. In the example shown in FIG. 4B, the height C of the rear cover is 4.6 mm as compared with a specification value of 4.0 mm when the rotation angle .gamma. is 35 degrees. Further, as shown in the graph in FIG. 5 it can be comprehended that the height C of the rear cover tends to approximate the specification value when the rotation angle (opening angle) .gamma. is within a range of 20 to 45 degrees.
The prior art tape cassette shown in FIGS. 4A and 4B has, however, a problem in which an inclined angle .theta. of an inclined portion 107a of the guide groove 107 for guiding the upper cover 104 is approximately 50 degrees with a large resistance caused when opening the cover body, and cuttings are produced in the guide groove and the guide spindles by this resisting force as the case may be. Such being the case, for reducing the resistance when opening the cover body, as shown in FIG. 6, if the inclined angle .theta. of the inclined portion 107a of the guide groove 107 is set stepwise smaller than 50 degrees, as shown in FIG. 7, the front cover opens at the rotation angle .gamma. within a range of 25 to 45 degrees, and the height C of the rear cover approximates the specification value. Then, when the inclined angle .theta. comes to 30 degrees, the specification value is not met. In the example shown in FIG. 6, when the rotation angle .gamma. of the front cover is 35 degrees, the height C of the rear cover is 4.2 mm as compared with the specification value of 4.0 mm at the inclined angle .theta. of 35 degrees.
Then, as shown in FIGS. 8A and 8B, if contrived so that the inclined angle of the inclined portion 108a of the cam groove 108 for guiding the rear cover 105 is increased and the rear cover 105 is moved fast up to a high position, in an example in FIG. 8B, when the rotation angle .gamma. of the front cover is 35 degrees, the height C of the rear cover is 4.7 mm for the specification value of 4.0 mm at the inclined angle .theta. of 30 degrees, which sufficiently satisfies the specification requirement. While on the other hand, however, as indicated by a trajectory x' of a front side end of the rear cover 105 in FIG. 8A, the rear cover 105 gets close to the tape and interferes with the tape with the result that the tape might undesirably be damaged.